Btrfs: dedupe_file_range ioctl: remove 16MiB restriction

Currently btrfs_dedupe_file_range silently restricts the dedupe range to
to 16MiB to limit locking and working memory size and is documented in
manual page as implementation specific.

Let's remove that restriction by iterating over the dedup range in 16MiB
steps.  This is backward compatible and will not change anything for
requests smaller then 16MiB.

Signed-off-by: Timofey Titovets <nefelim4ag@gmail.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/ioctl.c

@@ -3128,11 +3128,14 @@ again:
 	return ret;
 }
 
+#define BTRFS_MAX_DEDUPE_LEN	SZ_16M
+
 static int btrfs_extent_same(struct inode *src, u64 loff, u64 olen,
 			     struct inode *dst, u64 dst_loff)
 {
 	int ret;
 	bool same_inode = (src == dst);
+	u64 i, tail_len, chunk_count;
 
 	if (olen == 0)
 		return 0;
@@ -3149,7 +3152,21 @@ static int btrfs_extent_same(struct inode *src, u64 loff, u64 olen,
 		goto out_unlock;
 	}
 
-	ret = btrfs_extent_same_range(src, loff, olen, dst, dst_loff);
+	tail_len = olen % BTRFS_MAX_DEDUPE_LEN;
+	chunk_count = div_u64(olen, BTRFS_MAX_DEDUPE_LEN);
+
+	for (i = 0; i < chunk_count; i++) {
+		ret = btrfs_extent_same_range(src, loff, BTRFS_MAX_DEDUPE_LEN,
+					      dst, dst_loff);
+		if (ret)
+			goto out_unlock;
+
+		loff += BTRFS_MAX_DEDUPE_LEN;
+		dst_loff += BTRFS_MAX_DEDUPE_LEN;
+	}
+
+	if (tail_len > 0)
+		ret = btrfs_extent_same_range(src, loff, tail_len, dst, dst_loff);
 
 out_unlock:
 	if (same_inode)
@@ -3160,8 +3177,6 @@ out_unlock:
 	return ret;
 }
 
-#define BTRFS_MAX_DEDUPE_LEN	SZ_16M
-
 ssize_t btrfs_dedupe_file_range(struct file *src_file, u64 loff, u64 olen,
 				struct file *dst_file, u64 dst_loff)
 {
@@ -3170,9 +3185,6 @@ ssize_t btrfs_dedupe_file_range(struct file *src_file, u64 loff, u64 olen,
 	u64 bs = BTRFS_I(src)->root->fs_info->sb->s_blocksize;
 	ssize_t res;
 
-	if (olen > BTRFS_MAX_DEDUPE_LEN)
-		olen = BTRFS_MAX_DEDUPE_LEN;
-
 	if (WARN_ON_ONCE(bs < PAGE_SIZE)) {
 		/*
 		 * Btrfs does not support blocksize < page_size. As a